"The disaster has struck, the army entered Otaiba," a fighter from the town told Reuters via Skype. "The regime has managed to turn off the weapons tap."

Rebels pulled out of Otaiba after more than five weeks of fighting, during which they accused the government of using chemical weapons against them on March 19 and April 9.

On April 12, Western diplomats toldAgence-France Press they had "hard evidence" that chemical weapons have been used by the Syrian army. On April 25, U.S. Defense Secretary Chuck Hagel said that the White House has evidence of chemical weapons use.

"Now the operations are well-planned and the objectives are precise," Rami Abdel, director of the Syrian Observatory for Human Rights, told AFP. "This is because Iranian officers are on the ground, leading operations, while new Iranian weapons conceived for this kind of battle are flowing in."

REUTERS/George Ourfalian

The recent counteroffensive has both stemmed rebel momentum in the south and led to state gains in the north and major cities in the country's west, which link the capital to Assad's coastal stronghold.

Otaiba is pivotal as it servesa gateway from Syria's south into the eastern rural suburbs of Damascus known as al-Ghouta.

"Now all the villages will start falling one after another, the battle in Eastern Ghouta will be a war of attrition," a rebel fighter in the area told Reuters via Skype.

Assad's previous strategy shift, to rely on air superiority and wait for the opposition to fracture, continues as towns are bombed and distress calls from rebels in Otaiba go unanswered by rival rebel units.

"To all mujahedeen (holy warriors): If Otaiba falls, the whole of Eastern Ghouta will fall ... come and help," part of the message sent to fighters said.